Catalytic cracking is accomplished by contacting hydrocarbons in a reaction zone with a catalyst composed of finely divided particulate material. The reaction in catalytic cracking, as opposed to hydrocracking, is carried out in the absence of added hydrogen or the consumption of hydrogen. As the cracking reaction proceeds, substantial amounts of coke are deposited on the catalyst. The catalyst is regenerated at high temperatures by burning coke from the catalyst in a regeneration zone. Coke-containing catalyst, referred to as “spent catalyst”, may be continually transported from the reaction zone to the regeneration zone to be regenerated and replaced by essentially coke-free regenerated catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams facilitates the transport of catalyst between the reaction zone and regeneration zone.
A regeneration zone typically comprises a regenerator vessel that includes an outer shell and an internal riser. The internal riser is typically attached to the outer shell. However, given the extreme high temperatures that occur inside of the regenerator vessel, the riser will expand. Therefore, typically an annulus or void is provided between the outer shell and the internal riser to offer a longer section of internal riser to accommodate the thermal gradient and add more flexibility for thermal stresses. In order to prevent catalyst from filling this annulus, one or more seals are used to keep the annulus relatively free from catalyst.
A primary seal may be disposed between the outer shell and the internal riser comprising a flexible material to allow the shell and the riser to expand at different rates while preventing catalyst from descending below the primary seal. Insulation is installed in the annulus above and below the primary seal to prevent catalyst entry and to mitigate heat transfer to the shell.
Insulation has a low heat transfer coefficient. A heat transfer coefficient characterizes the ability to transfer heat across a medium by conduction, convection and radiation. Insulation typically comprises fibrous material that traps air in interstices between the fibers rendering the air stagnant and insulative against heat transfer. Air is not highly conductive, and it cannot be convective when it is stagnant. Hence, insulation has a low heat transfer coefficient.
There remains a need for an effective and efficient design for sealing an annulus between an internal riser and an outer shell in a catalyst regenerator.